nfsd: Don't reset the write verifier on a commit EAGAIN

If fsync() is returning EAGAIN, then we can assume that the filesystem
being exported is something like NFS with the 'softerr' mount option
enabled, and that it is just asking us to replay the fsync() operation
at a later date.

If we see an ESTALE, then ditto: the file is gone, so there is no danger
of losing the error.

For those cases, do not reset the write verifier. A write verifier
change has a global effect, causing retransmission by all clients of
all uncommitted unstable writes for all files, so it is worth
mitigating where possible.

Link: https://lore.kernel.org/linux-nfs/[email protected]/
Signed-off-by: Trond Myklebust <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

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